The present invention relates to a pneumatic, e.g. pressurized air operated, control system for shifting the auxiliary section of a compound transmission. In particular, the present invention relates to an improved pneumatic control and interlock system for a range type compound transmission.
Compound transmissions of the range or combined range/splitter type are well known in the prior art. Such transmissions typically comprise a multiple speed main transmission section connected in series with a range type auxiliary section wherein the range step is greater than the total ratio coverage of the main transmission section.
In such transmissions, the main section is typically shifted by means of a shift bar housing assembly controlled by a manually operated shift lever or the like. In contrast, the auxiliary range section is shifted by means of a button or switch, usually manually operated, that controls a remote slave valve/actuator mechanism. The valve/actuator mechanism usually controls operation of a range selection cylinder. The range selection cylinder includes a fluid-actuated piston that divides the cylinder into a low-range and a high-range chamber. The piston is made to move in response to selective pressurization of one chamber while simultaneously exhausting a second chamber. Since the range section often utilizes synchronized jaw clutches a range shift should be initiated and completed while the main transmission section is in neutral, to provide acceptable shift quality and to prevent undue wear and/or damage to the synchronized jaw clutches.
Prior art compound range type transmissions usually include a control system, which is usually a pneumatic control system having interlock devices, that allows a range shift to be preselected using a selector button or switch at a master control valve. To prevent damage to the jaw clutches, the control system prevents shift initiation until the main transmission section is shifted to, or at least towards, the neutral condition. One method for preventing shift initiation utilizes mechanical type interlock devices on the range section actuator mechanical linkage that physically prevent movement of the range section shift fork until the main section is shifted into neutral. A second, more common method of preventing shift initiation utilizes logic-based interlock devices of the type wherein the valve supplying pressurized fluid to the range section cylinder is either disabled or not provided with pressurized fluid until a shift to main section neutral is sensed, or is only activated and provided with pressurized fluid while the main section is shifted to and remains in neutral. Examples of such transmissions and the control systems therefor may be seen by reference to U.S. Pat. Nos. 2,654,268; 3,138,965, 4,060,005 and 4,974,474,the disclosures of which are hereby incorporated by reference.
In general, prior art pneumatic control systems bias the range section piston toward the low range position until a high range shift is requested, thereby preventing the situation where the vehicle operator starts while in high range. Then, when a high range shift is requested, the range section piston is biased to the high range selection position. When biased towards high range, the high range chamber of the range section piston is typically pressurized with compressed air to approximately 60 to 80 psi, while the low range chamber is exhausted. In those systems where the range section piston is not provided with compressed fluid until a shift to main section neutral, problems may occur wherein the vehicle operator completes a main section shift, including a traverse through main section neutral, before the high range chamber of the range section cylinder is able to pressurize and complete the range section shift from low to high range. In this situation, even though the range shift has not completed, the high range chamber of the range section piston remains pressurized, exerting high stress against the mechanical interlocks and exerting a significant force on the synchronizer and clutch jaws. Excessive force may lead to adverse consequences such as severe range section shifting, re-engagement of the main transmission section prior to completion of the range section shift, damage to the range section synchronizers (especially during an upshift) or stranding the range section in a disengaged condition. Additionally, when the range section is finally shifted into high range, prior art logic systems continue to pressurize only the range cylinder high range chamber while leaving the low range chamber exhausted, such that the range section piston continuously exerts a shifting force even after the shift to high range is completed and the main transmission is in gear. Such a continuous force may affect reliability of the range section transmission over the life of the vehicle.
Therefore, a new logic system is needed to reduce the force exerted by the range section piston when a high range shift is requested by the vehicle operator, but is blocked because the main transmission section is in gear, and when a high range shift is accomplished and the new transmission section is in gear.
The above disadvantages are minimized or overcome by utilizing a pneumatic control system including a first actuator valve that selectively connects a first range cylinder chamber to a pneumatic source when a high range shift is requested by the vehicle operator, and a second valve that selectively connects a second range cylinder chamber to a pneumatic source when the main transmission is in gear, but exhausts the second chamber when the main transmission section is in neutral. Preferably, both the first and second valves are housed within the same valve body.
According to the present invention, the second valve is a two part, two-position valve movable by means of a spring-biased plunger. The plunger is positioned within the transmission to interact with a pin the moves in response to shifting of the main transmission into gear or into neutral. In the preferred embodiment, the plunger interacts with a neutral indicating device on the pin to force the second valve between an engaged and a disengaged position. In the engaged position, the second valve connects the second range cylinder chamber to a pneumatic source. In the disengaged position, the second valve exhausts the second range cylinder chamber. To accomplish movement between engaged and disengaged positions, the pin includes a circumferential notch indicative of the main transmission section neutral position. When a detent on the plunger senses the notch, the spring-biased plunger moves into the notch, thereby moving the second valve from the engaged to the disengaged position. However, when the detent is not within the notch, the second valve is forced into the engaged position.
Thus, upon sensing that the main transmission section has moved from an in gear position to a neutral position, the second chamber is exhausted while the first chamber remains pressurized, thereby allowing the range actuator piston to move to a preselected position. Once in gear, however, the control system equalizes the pressures between the two chambers.
Therefore, in the situation where the main transmission section is in gear but the auxiliary transmission section has not completed a range shift, force exerted by the range actuator piston is substantially eliminated until the main transmission shifts back towards neutral. Also, since compressed air is already behind the actuator piston (i.e. the first chamber is pressurized) when the main transmission section moves into neutral, a very fast range change is achieved once the second chamber is exhausted. However, since the second chamber was pressurized, exhausting the second chamber prevents too harsh of a shift that could result in a range section synchronizer malfunctioning.
Moreover, once the range actuator piston reaches the preselected position and the main transmission is moved once again into gear, the second valve pressurizes the second cylinder chamber, thereby reducing the force continuously exerted by the range actuator piston when in gear and in high range, thereby improving reliability and decreasing wear over the life of the vehicle.